The present invention relates to a method for producing xcex1-sulfofatty acid alkyl ester salts which can produce xcex1-sulfofatty acid alkyl ester salts having a pale color close to white and preferably having less odor by a simple production process.
xcex1-Sulfofatty acid alkyl ester salts are used as surfactants, have a high cleaning power, have excellent biodegradability and have less adverse effects on the environment so that their ability as materials for detergents is highly appreciated. xcex1-Sulfofatty acid alkyl ester salts (hereinafter, sometimes referred to as xcex1-SF) are obtained by bringing fatty acid alkyl esters into contact with a sulfonating gas to sulfonate them to produce xcex1-sulfofatty acid alkyl esters and then neutralizing them.
FIG. 10 is a flowchart illustrating an example of the conventional production process for xcex1-sulfofatty acid alkyl ester salts.
In this example, the step of sulfonating raw material fatty acid alkyl esters, for example, comprises the step of introducing a sulfonating gas to bring about contact therewith and the aging step of holding them at a predetermined temperature for a predetermined time.
That is, when bringing a raw material into contact with a sulfonating gas, first the reaction to insert SO3 into the alkoxy group takes place to produce an SO3-mono-adduct, which further reacts with SO3, introducing a sulfonic group at the xcex1-position to produce an SO3-di-adduct and finally the SO3 inserted to the alkoxy group is eliminated to produce xcex1-sulfofatty acid alkyl ester, as shown in the following general formula (I). 
The sulfonating gas and raw material react in equimolar ratios theoretically. However, actually, the reaction is a gas-liquid reaction and since the reaction is a consecutive reaction, the sulfonating gas is used in excess of the equimolar amount. In this example, a sulfonating gas containing SO3 in a molar amount of, for example, 1.2-fold as compared with the raw material is used.
Then, since the reaction is a consecutive reaction as stated above, the reaction mixture after contact with the sulfonating gas contains a mono-adduct, a di-adduct, unused fatty acid alkyl ester, and other by-products.
For this reason, in the subsequent aging step, the elimination of SO3 from the di-adducts is promoted to finally attain an equilibrium state. Thus, in the sulfonated product that reaches the equilibrium state in the aging step, there exist di-adducts in an amount corresponding to the excessive mole number of SO3 at least relative to the raw material.
Neutralization of the di-adducts gives rise to xcex1-sulfofatty acid alkyl ester di-alkali salts which would not contribute to the washing effect, so that for detergent applications, it is necessary to reduce the content of the di-aduct as far as possible.
Hence, after the aging step, a lower alcohol is added to convert the di-adduct to an xcex1-sulfofatty acid alkyl ester according to the reaction shown by the general formula (II). As will be understood from the formula, since an xcex1-sulfofatty acid alkyl ester is produced from a di-adduct, this treatment is called an esterification step. 
In the sulfonation of a fatty acid alkyl ester, the xcex1-sulfofatty acid alkyl ester becomes colored, so that it is necessary to conduct a bleaching treatment. As illustrated in FIG. 10, the bleaching treatment is performed simultaneously with the esterification step by the addition of a bleaching agent such as an aqueous hydrogen peroxide solution together with the lower alcohol (hereinafter, the simultaneous esterification and bleaching step will be described).
In this case, theoretically, an equimolar amount of the lower alcohol reacts with the di-adduct in the sulfonated product. However, due to the side reaction by the action of the bleaching agent, the lower alcohol is consumed in an amount more than is used in the esterification. As a result, in this example, where methanol is used as the lower alcohol, methanol is used in an excess amount such as 30% by weight with respect to the sum of the xcex1-sulfofatty acid alkyl ester and the di-adduct (hereinafter, sometimes referred to as xe2x80x9csulfonic acid,xe2x80x9d or simply xe2x80x9cacidxe2x80x9d).
Also, in this example, 3% by weight of hydrogen peroxide is used in terms of pure compound with respect to the sulfonic acid.
Subsequently, the sulfonated product after the esterification and bleaching is neutralized with an aqueous alkali solution to obtain a neutralized product (xcex1-sulfofatty acid alkyl ester salt).
In this example, the concentration of the aqueous alkali solution is 20% by weight and the neutralized product is obtained as an aqueous slurry having an activator (AI) concentration of about 50% by weight. The AI (active ingredient, i.e., activator) is the sum of the xcex1-sulfofatty acid alkyl ester salt and xcex1-sulfofatty acid di-alkali salt.
Incidentally, the viscosity of an aqueous slurry of an anionic surfactant shows a unique behavior such that it increases proportionally with increases in the activator concentration in the range where the activator concentration is low but it decreases in a specified concentration range when the activator concentration increases to some extent and then it increases again when the concentration becomes higher than the specified concentration range. As a result, from the viewpoints of production efficiency and reduction in viscosity, it is preferable that the above neutralized product be adjusted to have an activator concentration (AI concentration) of 60 to 80% by weight where low viscosity is shown. To this end, the concentration of the aqueous alkali solution used is high enough to achieve such an AI concentration.
On this occasion, the presence of a lower alcohol can prevent the production of by-products such as xcex1-sulfofatty acid di-alkali salts by the following two actions.
That is, the viscosity of the above neutralized product can be further reduced by means of the lower alcohol, and the generation of by-products due to localized heating caused by the neutralization heat can be prevented. This is particularly effective where high concentration aqueous alkali solutions are used since in such cases side reactions tend to occur.
The presence of a lower alcohol also prevents generation of an xcex1-sulfofatty acid, which is a precursor of the xcex1-sulfofatty acid di-alkali salt, from the xcex1-sulfofatty acid alkyl ester by a reversible reaction in the above general formula (II).
To this end, in the neutralization step, a certain excess amount of lower alcohol is needed.
Therefore, the neutralized product contains an excess amount of lower alcohol. This makes it necessary to concentrate the above neutralized product to recover the lower alcohol, purify it remove water and recycle it to the esterification step as described above. The water separated from the lower alcohol by the purification is subjected to waste water treatment and then disposed of.
In this manner, the neutralized product now free of the lower alcohol is further formed into powder, particles, etc. by conventional methods to obtain commercial products.
On the other hand, xcex1-sulfofatty acid alkyl ester salts usually give perceptible odors so that in the case of detergent compositions in which xcex1-sulfofatty acid alkyl ester salts are blended, and formulations which improve the odor, for example, by masking with a perfume are used.
In addition, Japanese Patent Application First Publication No. Hei 08-081694 discloses that odors can be improved by use of fatty alkyl esters having an iodine value of 2 or less and a carotenoid content of 10 ppm or less as raw materials. Also, Japanese Patent Application First Publication No. Hei 07-197080 discloses that odors can be improved by blending zeolite in detergent compositions.
However, the above-described conventional production methods have the problem that excessive lower alcohols are used therein and hence, it is necessary to recover, purify and recycle the lower alcohols from the viewpoint of cost reduction, etc., which makes the production process complicated. Also, there is the problem that the bleaching treatment alone gives insufficient color improving effects.
In the improvement of odors, the problem arises that the formulation of detergent compositions is limited and the production process is made complicated by the use of treatments such as the distillation of raw materials, hydrogenation, solvent extraction, centrifugation, etc.
Therefore, an object of the present invention is to provide a method for producing xcex1-sulfofatty acid alkyl ester salts which includes a more simplified production process than conventional method.
More specifically, it is an object of the present invention to provide a method for xcex1-sulfofatty acid alkyl ester salts which requires no recovery, purification or recycling of lower alcohols.
Further, it is an object of the present invention to provide a method for xcex1-sulfofatty acid alkyl ester salts which can improve the color of the xcex1-sulfofatty acid alkyl ester salts.
Furthermore, it is an object of the present invention to provide a method for xcex1-sulfofatty acid alkyl ester salts which can improve the odor of the xcex1-sulfofatty acid alkyl ester salts.
The method for xcex1-sulfofatty acid alkyl ester salts according to the present invention comprises a sulfonation step for bringing a fatty acid alkyl ester into contact with a sulfonating gas in the presence of a coloring inhibitor to sulfonate the fatty acid alkyl ester, an esterification step for esterifying the product of the sulfonation step with a lower alcohol, a neutralization step for neutralizing the esterified product of the esterification step to obtain a neutralized product, and a bleaching step for bleaching the neutralized product to obtain a bleached product.
In the method for xcex1-sulfofatty acid alkyl ester salts, it is preferable that a deodorization step for deodorizing the bleached product be conducted.
Further, in the above deodorization step, it is preferable that the bleached product be deodorized by a flush method.